In recent years, fiber lasers have been considered to be serious alternatives to solid state and CO2 lasers for military, aerospace, medical and industrial material processing applications. Fiber lasers are very attractive due mainly to their high output power as well as excellent beam quality and the flexibility of the design of the lasers. Large mode area double-clad fibers (LMA-DCF) are one of the key components in fiber lasers. In order to efficiently couple pump-energy into such an LMA-DCF and also to allow a high output power, the cladding of the LMA-DCF is designed to have a high numerical aperture and variously shaped cross-sections, e.g. round, octagon, square etc. The cladding diameters of LMA-DCFs are typically in the range of 300-1000 μm, depending on the level of the output power. The cores of LMA-DCFs are often doped with a high concentration of rare-earth elements, e.g. ytterbium, and their sizes can be as large as 50 μm with a low numerical aperture to reduce non-linear effects.
One of the major problems that deter the use of LMA-DCFs is the fact that it is very difficult to achieve high quality splices of such fibers using traditional splicing techniques. Due to general demands in the design of fiber lasers, fibers of different types have to be spliced to each other, e.g. splices between differently shaped LMA-DCFs, such as a round LMA-DCF spliced to an octagon LMA-DCF, an LMA-DCF containing rare-earth dopants spliced to an LMA-DCF not containing rare-earth dopants, and an LMA-DCF spliced to a conventional single mode optical fiber (SMF), the latter two fibers cladding diameters that differ very much from other, e.g. by a factor of three or more. The major difficulty in splicing LMA-DCF is the failure of traditional core-alignment processes which are used in conventional fusion splicers. Two primary problems can be observed. First, the information on cladding edges requested by traditional alignment processes could not be fulfilled since the size of LMA-DCF is too large to be handled by the imaging system used in conventional splicers. Second, it is difficult to simultaneously observe core images of two optical fibers for which there is a huge difference as to their cladding diameters and their structures. Thus, in practice, manual alignment processes assisted by power transmission measurements are often used to splice LMA-DCFs, resulting in a low efficiency of the manufacturing process and a low yield since the splices often have a too low quality.
The development of the conventional core alignment processes used today can be traced back to pioneering work two decades ago, cf. T. Katekuri et al., IEEE J. Lightwave Technol., Vol. 2, pp. 277-283, 1984. These core alignment processes are based upon analysis of core images extracted from light intensity profiles of the fibers to be spliced. In such processes, a core image of a considered fiber is obtained by illuminating the fiber from the side thereof using an external light source. It has been demonstrated theoretically as well as experimentally that the core image of a fiber can be resolved by placing the object plane of a high resolution imaging system near the fiber edge, as seen from the imaging system, where the light rays leave the fiber. Using information extracted from the core image, various automatic core alignment processes have been developed.
One of the core alignment processes based on image analysis is disclosed in various Japanese patents, see e.g. the Japanese patent 11194227 for Fujikura. Using these processes, in the pictures taken of fibers to be spliced, the vertical distance between the positions of the e.g. upper edge of the cladding and of the approximate center of the core image is measured for each fiber, the fibers as conventional assumed to be located horizontally in the pictures. The alignment is performed by then displacing the two fibers in relation to each other so that the difference of said two measured distances of the two fibers becomes equal to the vertical difference between the positions of the upper edges of the claddings of the two fibers. Since this method relies on the information extracted from both the core images and images of the edges of the cladding, it is difficult to perform an accurate core alignment. Due to the significant differences in regard of refractive indices, light passing only through the claddings behaves differently compared to the light passing through both the cladding and the core. Thus, the optimum position of the object plane to get core images of a high quality is not equal to the optimum position to get images of the cladding edges that have a high quality. This fact implies that it may not be possible to simultaneously measure the positions of the core and the cladding edges of a fiber with a high accuracy, this in turn resulting in a degradation of the alignment accuracy when based on such pictures. The need for information about the position of the cladding edges in the alignment process also results in a need for special imaging systems including huge sensors that are very expensive and hence may not be cost effective in the manufacture of splicers.
A different method using so-called warm-fiber image analysis for core alignment is disclosed in e.g. U.S. Pat. No. 5,570,446 assigned to Ericsson. In this method, instead of illuminating the fibers with external light, an electric glow discharge giving a relatively low fusion temperature is used to heat the ends of fibers to be spliced before actually making the ends contact each other. Since the dopant concentration in the core of a fiber usually is much higher than that in the cladding, the thermal light emission from the core is much stronger than that from the cladding, this resulting in a picture in which there is a core image of the hot or warm fiber. By carefully analyzing a light intensity profile derived from such a warm-fiber picture, information on the position of the core of the fiber can be extracted for use in the core alignment process. Since this method does not require information on the positions of the edges of cladding, it is possible to perform the core alignment process with a high accuracy. It is found, however, that in the pre-heating step process used in this method it is very difficult to observe the core images in pictures of LMA-DCFs. This is because the energy needed for heating the cores of LMA-DCF fibers is much higher than that needed for conventional optical fibers used for communication, this fact resulting in thermal light emission that usually causes saturation of the imaging system of convention fusion splicers. An additional problem is the diffusion of the core dopants occurring in the pre-heating step. This diffusion can cause a significant expansion of the optical mode field diameter (MFD) and result in an MFD mismatch of the two fibers at the splice point, which may in turn give high optical losses in the splice.
Therefore, there is a need in the art to develop a method that can avoid the drawbacks of the existing techniques so that core alignment processes of a high accuracy can be performed for fibers of all types, particularly for LMA-DCFs.
A method of recentering the plane in which pictures are captured is disclosed in the published International patent application No. WO 01/86331, “Arc Recentering”, wherein the capturing plane is moved dependent on the center of the electric arc.